Touch probes with movable arm-sets that carry feelers are utilized in coordinate measuring machines and in machine tools, more specifically in machining centers and lathes, for carrying out checkings on machined or to be machined pieces, on the tools, on the machine tables, etc. In each of these probes, contact between feeler and, for example, a mechanical piece is monitored by suitable devices which detect specific displacements of the movable arm-set with respect to a casing and control the reading of transducers associated with the machine slides, that provide measurement values with respect to a reference position or origin.
A probe detecting and monitoring device can foresee the utilization of an electric circuit and of (at least) an associated switch that, actuated in a mechanical way as a result of displacements occurring between the movable arm and the casing, causes the closure or, more frequently, the opening of the circuit.
The U.S. Pat. No. 5,299,360 discloses probes according to the preamble of claim 1, each including a microswitch with a stem having a free end that cooperates with the movable arm-set. More specifically, in each of said probes, the coupling between the movable arm-set and the stationary casing is such that, further to displacements of the feeler in a longitudinal direction or in a transversal direction, there occurs the lifting of an abutment surface of the movable arm-set that is near the microswitch and the consequent thrust on the stem, the disengagement of the contacts and the opening of the electric circuit.
An extremely important characteristic that touch probes are required to have is a high standard of repeatability i.e. a close correlation between specific positions taken by the feeler and the opening of the electric circuit.
In order to improve said characteristic, the mutual arrangement among the component parts of the switch is accurately defined insofar as, among other things, the aligning—along a longitudinal axis of the thrust spring—of the movable contact, generally ball shaped, and of the stem is concerned. Although the probes disclosed in U.S. Pat. No. 5,299,360 guarantee good repeatability, an absolutely perfect alignment between the component parts of the switch is not however possible. Furthermore, variations in the mutual positions of the stem and of the other component parts of the microswitch due to clearances in the longitudinal guide system and/or possible rotations of the stem about its axis, even though of extremely small entity, can negatively affect the repeatability of the probe. This is especially true in recent high precision applications in which it is required that the repeatability errors be particularly small, considerably smaller than 1 μm.
Other inconveniences that occur in the known probes as, for example, those disclosed in the formerly mentioned U.S. Pat. No. 5,299,360, but not just in those probes, hang on the reliability of the electric contacts of the microswitch. In fact, although the probes are protected by gaskets of various types, they cannot be, in general, considered hermetically sealed, more particularly the rubber gaskets are not entirely sealed against oxygen and water vapour. These two elements, oxygen and water vapour, put together or separately, concur to oxidation processes of the microswitch electric contacts and hence lead to cause malfunctionings, that can affect the reliability of the probe. In order to avoid the occurring of these negative processes, it is known to utilize, at the interior of the microswitch, lubricating fluid (“oil”) for protecting the contacts. However, although the presence of oil considerably improves the reliability of the contacts, it can interfere with the metrological performance of the probe, in particular negatively affect its repeatability. In fact, the instant when the opening of the contacts—that are fed with very low voltages—is detected can be altered in an unforeseeable way owing to the presence of oil on the contacts.